<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        input:not(:focus) + .callout { transform: scale(0);
        }
        .callout {
        transition: .5s transform; transform-origin: 1.4em -.4em;
        }
    </style>
</head>
<body>
    <label>
        Your username: <input id="username" />
        <span class="callout">Only letters, numbers, underscores (_) and hyphens (-) allowed!</span>
    </label>
</body>
</html>